Current- and voltage-controlled oscillators (ICO and VCO) are important components in the structures of transmitters and receivers. When applications to portable wireless communications systems are concerned, the main requirements for VCO/ICOs are: an operational frequency range of 1 to 20 GHz, a very low phase noise and the lowest possible operating voltage and power consumption. Depending on the structure, a communications device may comprise several VCO/ICOs needed for different purposes, e.g. frequency conversion, synthetization, modulation, etc. Their performance affects strongly the performance of the entire communications unit. On the other hand, the demand to implement these oscillators for silicon technologies faces several problems.
During the last few years several research projects have focused on finding optimal solutions. Two types of oscillators are mainly used as the cores of VCO/ICOs: sinusoidal oscillators and relaxation oscillators. Sinusoidal oscillators usually produce the best parameters as far as high frequency and low phase noise are concerned, but they can be easily implemented mostly on GaAS technologies only. A transition to bipolar, CMOS or BiCMOS technologies causes several problems mainly due to the highly conductive substrate. On the other hand, the speed of such available technologies is a challenge to researchers, as at present transient frequencies of 10 to 40 GHz are reached, which was previously considered to be a transient range possible to be covered only by materials based on GaAS. The speed of silicon-based technologies is sufficient enough for mobile communication in the frequency range of 1 to 20 GHz, used by most mobile stations and wireless LANs. An additional driving factor in the design of portable equipments has always been a high demand for as low an operating voltage as possible and a very low power consumption.
In oscillators of LC type, the active circuit components are kept out of the non-linear operation range, whereas in relaxation oscillators, the sinusoidal signal is the result of the incapability of the pulse circuit to switch fast enough at very high frequencies.
Oscillator circuits, i.e. oscillators, can be implemented by many different circuit structures. One of them is an astable (free-running) multivibrator. FIG. 1 shows a conventional emitter-coupled multivibrator circuit, which has been used for implementing voltage-controlled oscillators (VCO). The circuit comprises two transistors Q1 and Q2, between which is provided a positive feedback by connecting each transistor collector via a buffer transistor Q3, Q4 to control the base of the other transistor. The collectors of Q1 and Q2 are connected via resistors Rc1 and Rc2, respectively, to one potential of an operating voltage source 1 and the emitters are connected via current sources 3 and 4, respectively, to the lower potential of the operating voltage source. Correspondingly, the emitters of the buffer transistors Q3 and Q4 are connected via current sources 5 and 6 to the lower potential. Additionally, a reference capacitor C is connected between the emitters of Ql and Q2. The positive feedback and series resonance circuits Rc1-C and Rc2-C constituted by the resistors RC1 and RC2 and the capacitance C lead to that the output of the multivibrator oscillates continuously between-two states, after the oscillation once has been trigged. The oscillation frequency is determined by the component values of the RC series resonance circuits. The oscillation frequency can be controlled by changing some of these component values, typically the capacitance C.
In the following, the operation of the multivibrator will be examined closer. To begin with, it is assumed that Q1 and Q3 are off (non-conduction state). When Q1 is off, the collector of Q1 and the base of Q2 are generally at the operating voltage potential. Then Q2 is on (conducting state), and its emitter current is I1+I2. The buffer transistor Q4 is likewise on and feeds base current to Q2. When Q2 is conductive, the current I1 flows from the emitter of Q2 via the capacitance C to the emitter of Q1. Then the current I1 charges/discharges the charge of the capacitance C, whereby the emitter potential of Q1 falls at a predetermined speed until Q1 becomes conductive when the base emitter voltage of Q1 exceeds about 0.6 V. When Q1 becomes conductive, its collector voltage begins to fall, which leads to that the buffer transistor Q3 begins to close. On account of a positive feedback via Q4, the base voltage of Q2 falls as well and Q2 closes. Q2 closing makes the collector voltage of Q2 rise, which accelerates the opening of Q3. Q3 opening increases the base current of Q1 via a positive feedback. A higher base current discharges parasitic capacitances of the base circuit of Q1 faster and accelerates thus the opening of Q1. When Q2 is off and Q1 is on, the current I2 flows from the emitter of Q1 via the capacitance C to the emitter of Q2, where the emitter voltage begins to fall until it makes Q2 open again and Q1 close via Q3.
The speed of such a multivibrator circuit (maximum resonance frequency) depends primarily on the properties of the transistors Q1 and Q2. The buffer transistors Q3 and Q4 increase the speed of the multivibrator circuit, because they make a higher base current possible, which again discharges the parasitic capacitances of the base circuit of the transistors Q1 and Q2 faster and accelerates thus the switching of the transistor from one state to another.
The lowest possible operating voltage Vcc will be achieved when it is assumed that the current sources 3 and 4 are ideal, i.e. no voltage loss is provided in them. When the ideal current sources are replaced by some practical circuit structure, such as current mirrors, Vcc increases. Returning to the operating principle of the circuit, it can be stated that current paths are either Q1-C-current mirror4 or Q2-C-current mirror 3 and that the current mirrors produce a stable current through the reference capacitor C, which is the main reason for the typical low phase noise. In search of a new way of increasing the speed, the reference capacitor cannot be decreased much more, because it will be of the order of parasitic capacitances, which leads to the fact that a controlled planning of the circuit is not possible.
Nowadays there is, however, a need of ever-increasing speeds while an operating voltage as low as possible is desired, especially in electronic equipments using battery power supplies.
For an implementation of a voltage- or current-controlled oscillator by means of a multivibrator circuit, the circuit requires a suitable supplementary control. Such a control should be as simple as possible.
In the circuit of FIG. 1, the pulse amplitude is determined by the sum of the currents I1+I2 multiplied by the value of the collector resistor Rc1 or Rc2 of the corresponding cycle. The pulse width is determined by the value of the current which is supplied by I1 or I2 via the reference capacitor C during its recharge cycles. Accordingly, either the capacitance of the reference capacitor C or the current flowing through it has to be changed for the frequency control.
The capacitance may be changed if a varactor is used as reference capacitor C. A problem is, however, that varactor technologies are not generally compatible with BiCMOS technologies, for instance. In the BICMOS technology, a PN junction can be used instead. But then the capacitor works in the circuit of FIG. 1 and changes continuously the polarity of the voltage. In this case, a serial connection of two varactors, opposite to each other, may be some sort of solution, but the operation of the forward voltage region of one diode shows certain non-linearities and the phase noise of the multivibrator could be so high that it is unacceptable.
Another alternative is to change the current and, in consequence of that, the recharge speed of the capacitor. This a very effective way of controlling the frequency of the oscillations, but the main drawback is its direct influence on the amplitude of the pulses.